When chip-like semiconductor devices are manufactured from a workpiece such as a semiconductor wafer, a blade dicing process has conventionally been performed in general to obtain chips by cutting the workpiece using a rotating blade while spraying the workpiece with a liquid for washing, etc. In recent years, however, a process of stealth dicing (registered trademark) is being adopted, which is a dry process that is capable of division into chips. In the process of stealth dicing (registered trademark), workpiece is irradiated with laser light having a large numerical aperture (NA) so that modified layers are formed preliminarily in the workpiece, while minimizing the damage which the workpiece receives in the vicinity of the surface, and thereafter in an expanding step or the like, chips are obtained by applying force to the workpiece.
As a specific example, the expanding step is performed in a manner as below. A sheet for semiconductor wafer processing is put into a state in which a workpiece that has been irradiated with laser light to form modified layers therein (also referred to as a “modified workpiece” in the present description) is attached to the sheet and a ring frame is attached to the circumference of the modified workpiece. A ring-like member is caused to come into contact with a region of the sheet for semiconductor wafer processing that is located between the region to which the modified workpiece is attached and the region to which the ring frame is attached, and the relative positions of the ring-like member and the ring frame in the vertical direction are varied. This varying the relative positions allows the dicing sheet to be applied with the tensile force. Varying the relative positions in the vertical direction as the above is ordinarily performed by pulling down the ring frame with respect to the ring-like member.
When a tensile force is applied to the sheet for semiconductor wafer processing in the expanding step as the above, loosening may occur in the sheet for semiconductor wafer processing to an extent that affects the subsequent steps. Specifically, if the loosening amount due to applying a tensile force to the sheet for semiconductor wafer processing (the displacement distance in the vertical direction of the bottom surface of the sheet for semiconductor wafer processing with reference to the lower side surface of a portion of the sheet for semiconductor wafer processing which is attached to the ring frame) is unduly large, the loosened bottom surface of the sheet for semiconductor wafer processing or the vicinity thereof will readily collide with foreign materials during transport, thus leading to poor handling ability when using the sheet for semiconductor wafer processing. Therefore, when the loosening amount of the sheet for semiconductor wafer processing is large, the sheet for semiconductor wafer processing may be partially heated to allow for the thermal shrinkage of a base film for sheets for semiconductor wafer processing, which is possessed by the sheet for semiconductor wafer processing, so that the loosening amount of the sheet for semiconductor wafer processing can be reduced. In the present description, the phenomenon that the loosening amount of the sheet for semiconductor wafer processing is reduced on the basis of the thermal shrinkage of the above base film may also be referred to as “recovery,” and a base film for sheets for semiconductor wafer processing capable of providing a sheet for semiconductor wafer processing that has at least one of the property of allowing for easy recovery and the property of having a large recovery amount (reduction amount of the loosening amount) may also be referred to as a “base film for sheets for semiconductor wafer processing that has recoverability.”
Patent Literature 1 discloses using a base film for sheets for semiconductor wafer processing that comprises a thermoplastic crosslinked resin having a thermal conductivity of 0.15 W/m·K or more, as a base film for sheets for semiconductor wafer processing that is excellent in the recoverability.